Preparation method of high-strength ultrathin battery diaphragm

Abstract

The invention discloses a preparation method of a high-strength and ultrathin battery diaphragm, which comprises the following steps: respectively weighing a mixture of polyolefin resin, a pore-forming agent and an additive in proportion, putting into a storage tank, simultaneously heating the storage tank in which the pore-forming agent is positioned, respectively feeding into a stirring tank, heating, premixing and stirring, feeding the mixed material into a feeding tank, feeding the mixed material into a double-screw blending extruder through the feeding tank for extrusion, forming material accumulation on the extruded material between a cooling roller with a water bath and an adjustable roller, and then extruding and casting the extruded material into a thick sheet through a gap between the cooling roller and the adjustable roller; sequentially carrying out longitudinal stretching and two-way synchronous stretching on the obtained thick sheet, and extracting and drying to obtain a semi-finished product diaphragm; transversely stretching the obtained semi-finished diaphragm for the second time to obtain a middle diaphragm; and carrying out longitudinal heat setting treatment on the obtained middle diaphragm to obtain the lithium battery diaphragm. The prepared battery diaphragm is high in strength, ultrathin in thickness and uniform in thickness and micropore aperture distribution.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery manufacturing, and in particular relates to a method for preparing a high-strength, ultra-thin battery separator. Background Art

[0002] The technological evolution of lithium-ion batteries has entered a phase of collaborative innovation encompassing materials, equipment, and applications. Driven by policy support, market demand, and scientific breakthroughs, their advantages are shifting from single-performance leadership to building system-level competitiveness. In the future, they will further develop advanced safety, ultra-fast charging, and all-weather adaptability, becoming a core pillar of the global energy transition. The EU's CBAM carbon tariff is forcing reductions in carbon emissions throughout the entire lifecycle, and China's battery companies' carbon footprint tracking system now covers 80% of their production capacity. Battery capacity density requirements are also increasing, requiring separators to become thinner while maintaining consistent mechanical properties.

[0003] Lithium-ion batteries, sodium-ion batteries, and condensed matter batteries are composed of a positive electrode material, a negative electrode material, an electrolyte solution (or condensed matter electrolyte), and an intermediate battery separator. All of these batteries require an intermediate separator, which primarily isolates the positive and negative electrodes and prevents electrons from passing through the battery while allowing ions to pass through, thereby enabling the rapid transfer of lithium ions between the positive and negative electrodes during electrochemical charge and discharge. Separators are a high-value-added material with the highest technical barriers in lithium-ion batteries (lithium batteries), accounting for approximately 15%-30% of the battery's cost. Separator permeability is determined by the membrane's pore size, pore size distribution, and porosity, and directly affects the battery's internal resistance to a certain extent. In the lithium battery structure, the separator is a key internal component, and its properties determine the battery's interface structure, internal resistance, and other factors, directly affecting the battery's capacity, cycling performance, and safety.

[0004] Battery separators must meet requirements for barrier properties, porosity, chemical stability, electrical stability, wettability, mechanical strength, flatness, and safety. Porosity determines the efficiency of lithium-ion transport. High porosity improves ionic conductivity, but excessively high porosity can reduce mechanical strength. Low porosity increases the battery's internal resistance, affecting charge and discharge performance; excessively high porosity can create a short-circuit risk. Separator thickness must balance mechanical strength and energy density. Thinner separators can increase battery volumetric energy density, but may compromise safety. Typical values: Commercial separator thickness is typically 7-16μm. Thermally stable separators must remain stable at high temperatures to prevent thermal shrinkage (for example, PE separators melt around 120°C, and PP at approximately 160°C). Thermal shrinkage can lead to direct contact between the positive and negative electrodes, causing short circuits or even thermal runaway. Puncture strength measures the separator's resistance to physical penetration and is directly related to battery safety. Typical values: Commercial separator puncture strength is typically 300-500gf (approximately 3-5N). Power battery separators usually require a puncture strength ≥400gf, while consumer batteries can be slightly lower.

[0005] However, the existing membrane preparation process has the problem of uneven dissolution of raw materials. Due to the uneven dissolution and phase separation between polyethylene material and mineral oil, the distribution of polyethylene in the formed lithium-ion battery membrane is uneven, which in turn makes the micropore distribution of the formed lithium-ion battery membrane uneven and the membrane thickness uneven, thus failing to meet the requirements of lithium-ion battery production.

[0006] Therefore, in order to solve the above-mentioned shortcomings, the present invention proposes a method for preparing a high-strength, ultra-thin battery separator. Summary of the Invention

[0007] The main technical problem solved by the present invention is to provide a method for preparing a high-strength, ultra-thin battery separator. The prepared battery separator has high strength, ultra-thin thickness, and uniform thickness and micropore diameter distribution.

[0008] To solve the above technical problems, the present invention adopts a technical solution: providing a method for preparing a high-strength, ultra-thin battery separator, comprising the following steps: S1: A mixture of polyolefin resin, pore-forming agent and additives in proportion is weighed and placed in a storage tank, and the storage tank containing the pore-forming agent is heated. The materials are then added to a stirring tank for heating and pre-mixing. The mixed materials are then sent to a feeding tank, and then sent to a twin-screw compounding extruder through the feeding tank for extrusion. The extruded materials are accumulated between a cooling roller with a water bath and an adjustable roller, and then extruded into a thick sheet through the gap between the cooling roller and the adjustable roller; S2: longitudinally stretching and biaxially stretching the obtained thick sheet in sequence, extracting and drying to obtain a semi-finished separator; S3: performing a second transverse stretching on the obtained semi-finished diaphragm to obtain a middle diaphragm; S4: performing longitudinal heat setting treatment on the obtained middle separator to obtain a lithium battery separator.

[0009] In a preferred embodiment of the present invention, in step S1, the mass ratio of the polyolefin resin to the pore-forming agent is 1:(3-5), the mass of the additive is 0.3%-2.5% of the mass of the pore-forming agent, the heating temperature of the storage tank containing the pore-forming agent is 95°C, the feeding rate of the polyolefin resin is 50 kg / h, the feeding rate of the mixture of the pore-forming agent and the additive is 200 kg / h, the stirring time of the stirring tank is 2 hours, the internal temperature of the twin-screw blending extruder is 200-260°C, the extrusion temperature is 200-260°C, the temperature of the water bath is 9-26°C, the temperature of the cooling roller is 7-30°C, and the thickness of the slab is 1-4 mm.

[0010] In a preferred embodiment of the present invention, the polyolefin resin is an ultra-high molecular weight polyolefin resin of 1.5-2.3 million or a high-density polyolefin resin.

[0011] In a preferred embodiment of the present invention, the pore-forming agent is phthalate, phosphate or white oil.

[0012] In a preferred embodiment of the present invention, the additive is one or more of an antioxidant, an antistatic agent or an inorganic filler.

[0013] In a preferred embodiment of the present invention, in step S2, the temperature for longitudinal stretching is 90-130°C, the stretching ratio is 1.2-5.0, the temperature for bidirectional synchronous stretching is 100-130°C, the longitudinal stretching ratio is 6.5-7.5, the transverse stretching ratio is 10-16, and dichloromethane is used for extraction.

[0014] In a preferred embodiment of the present invention, in step S3, the temperature of the second transverse stretching is 100-135° C., and the stretching ratio is 1.1-2.

[0015] In a preferred embodiment of the present invention, in step S4, the temperature of the longitudinal heat setting equipment is 80-100°C, and the stretching ratio is 0.8-1.

[0016] The beneficial effects of the present invention are as follows: the present invention provides a method for preparing a high-strength, ultra-thin battery separator, and the prepared battery separator has high strength, ultra-thin thickness, and uniform thickness and micropore diameter distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive work, among which: Figure 1 The present invention is a process flow chart of a preferred embodiment of a method for preparing a high-strength, ultra-thin battery separator. DETAILED DESCRIPTION

[0018] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] See also Figure 1 As shown, the embodiment of the present invention includes: A method for preparing a high-strength, ultra-thin battery separator comprises the following steps: S1: A mixture of polyolefin resin, pore-forming agent and additives in proportion is weighed and placed in a storage tank, and the storage tank where the pore-forming agent is located is heated to 95°C. The materials are then fed into a stirring tank for heating and pre-mixing. The feeding rate of the polyolefin resin is 50 kg / h, and the feeding rate of the mixture of the pore-forming agent and additives is 200 kg / h. The stirring time of the stirring tank is 2 hours. The mixed materials are fed into a feeding tank, and then fed into a twin-screw blending extruder through the feeding tank for extrusion. The extruded materials form a material accumulation between a cooling roller with a water bath and an adjustable roller, and then are extruded into a thick sheet through the gap between the cooling roller and the adjustable roller; S2: The obtained thick sheet is sequentially subjected to longitudinal stretching and biaxial synchronous stretching, and a semi-finished diaphragm is obtained after extraction and drying; S3: The obtained semi-finished diaphragm is subjected to a second transverse stretching to obtain an intermediate diaphragm; S4: The obtained intermediate diaphragm is subjected to longitudinal heat setting treatment to obtain a lithium battery diaphragm.

[0020] Among them, such as Figure 1 As shown, polyolefin resin is placed in storage tank 1, a mixture of pore-forming agent and additives is placed in storage tank 2, a stirring tank 3 is used to mix the materials in storage tank 1 and storage tank 2, a feeding tank 4 is used to feed the mixed materials into a twin-screw blending extruder 5, and a G1 cooling roller and a G2 extrusion roller are correspondingly provided in the water bath A. The material extruded by the twin-screw blending extruder 5 is accumulated on the G1 cooling roller and the G2 extrusion roller.

[0021] In step S1, the polyolefin resin is an ultra-high molecular weight polyolefin resin or a high-density polyolefin resin of 1.5-2.3 million, preferably polyethylene or polypropylene, more preferably a polyolefin resin with a number average molecular weight of 15-23×10 5 g / mol of polyethylene.

[0022] The pore-forming agent is a pore-forming agent well known to those skilled in the art and is not particularly limited. It is preferably phthalate ester, phosphate ester or white oil, and more preferably white oil.

[0023] The additives are any additives well known to those skilled in the art without any special restrictions. They are preferably one or more of antioxidants, antistatic agents or inorganic fillers, preferably antioxidants. The antioxidants are any antioxidants well known to those skilled in the art without any special restrictions. They are preferably 2,6-di-tert-butyl-4-methylphenol or polyhydric hindered phenol macromolecular antioxidant IRGANOX1010, more preferably polyhydric hindered phenol macromolecular antioxidant IRGANOX1010.

[0024] Polyolefin resins and additives promote phase separation in separators: Additives can adjust the thermodynamic and kinetic properties of the casting solution, prompting phase separation during solidification and forming a separator with a specific pore structure. Controlling pore size and distribution: By selecting appropriate additives and their dosage, the pore size and distribution of the separator can be effectively controlled; mechanical properties can be improved, tensile strength can be increased, and flexibility can be improved; thermal stability can be provided; surface hydrophilicity can be optimized; and ion conductivity can be increased. Certain additives can form channels within the separator's microporous structure that are conducive to lithium ion transport and regulate electrolyte distribution. Additives can affect the distribution and retention of the electrolyte in the separator. Additives can be selected based on the requirements of the battery separator's final use.

[0025] The mass ratio of the polyolefin resin to the pore-forming agent is 1:(3-5), preferably 1:(3-4).

[0026] The mass of the additive is 0.3%-2.5% of the mass of the pore-forming agent, preferably 0.5%-1%.

[0027] The ratio of polyolefin resin, pore former and additives, as well as the stability of feed and uniformity of premixing, are directly related to the pore structure and distribution: the ratio of pore former directly affects the porosity, pore size and distribution uniformity of the final product. An unbalanced ratio may result in pores that are too large, too small or unevenly distributed, affecting the key properties of the material (such as air permeability, filtration efficiency or liquid absorption capacity). Mechanical properties: Polyolefin resin is the main source of material strength. If its ratio is insufficient or the mixing is uneven, it may lead to a decrease in material strength (such as insufficient tensile strength and toughness). Functional properties: The ratio and dispersion of additives (such as antioxidants and flame retardants) will affect the material's weather resistance, thermal stability or flame retardancy. For example, uneven distribution of antioxidants may lead to accelerated local aging.

[0028] The internal temperature of the twin-screw blending extruder is 200-260°C, preferably 190-230°C, and the extrusion temperature is 200-260°C, preferably 200-230°C.

[0029] The extrusion method used in the twin-screw blending extruder can be any extrusion method familiar to those skilled in the art. The configuration of the twin-screw blending extruder can directly affect the properties of the extruded slab and the final performance of the diaphragm. In the production of polyolefin diaphragms, additives such as antioxidants are often added to improve the performance of the diaphragm. After the pre-mixing process, the powerful mixing capability of the twin-screw blending extruder can be used to evenly mix the additives with the polyolefin resin, ensuring that the additives are evenly distributed throughout the polyolefin resin, thereby achieving consistent performance across different parts of the diaphragm.

[0030] The temperature of the water bath is 9-26°C, preferably 12-18°C, and the temperature of the cooling roller is 7-30°C, preferably 9-22°C, more preferably 12-18°C.

[0031] Before extrusion, the extruded material is first piled up between the cooling roller with a water bath and the adjustable roller. The material is then extruded and cast into a thick sheet. The gap between the two is small, so the material first piles up and then is extruded and cast into a thick sheet through the gap between G1 and G2. This avoids the uneven thickness of the extruded sheet caused by the unstable pressure of the twin-screw blending extruder and the deviation in the amount of extruded material. At the same time, the solid mixture of polyolefin and pore-forming agent is crystallized in water bath A. The water cooling makes the front and back of the polyolefin crystals more uniform, resulting in a more uniform thickness of the final diaphragm. The faster cooling speed of the cooling roller can reduce the crystallinity of the sheet and prevent the formation of crystal balls. It can also cause thermotropic phase separation between the polyolefin resin and the pore-forming agent. The rapid cooling of the sheet surface locks most of the pore-forming agent that has already phase-separated inside the sheet, making it difficult for the pore-forming agent to flow away or seep out.

[0032] The thickness of the slab formed by extrusion casting can be determined according to the final purpose of the diaphragm and the stretching ratio, and there is no special limitation. In the present application, the thickness of the slab is 1-4 mm, preferably 2-3 mm.

[0033] In step S2, the longitudinal stretching temperature is 90-130°C, preferably 95-115°C, and the stretching ratio is 1.2-5.0, preferably 1.2-3. During this process, the shape of the micropores occupied by the pore-forming agent will be elongated in the longitudinal direction, and the size of the pores is controlled by the stretching temperature. At the same time, the thick sheet also obtains mechanical strength in the longitudinal direction.

[0034] The temperature for biaxial synchronous stretching is 100-130° C., preferably 110-130° C., the longitudinal stretching ratio is 6.5-7.5, preferably 7.5, and the transverse stretching ratio is 10-16, preferably 11-15.

[0035] The temperature of biaxial stretching is controlled by hot air heating, and convection heating improves fluidity, ensuring uniform heating of the film. Biaxial stretching elongates the film's micropores in both the longitudinal and transverse directions. After undergoing both longitudinal and biaxial stretching, the film achieves high mechanical strength in both directions, while also ensuring the film's air permeability and puncture resistance meet the requirements for power battery use.

[0036] The extraction is performed using dichloromethane. The extraction is a method well known to those skilled in the art and is not particularly limited. The pore-forming agent is removed by extraction to obtain a semi-finished diaphragm.

[0037] In step S3, the temperature of the second transverse stretching is 100-135° C., preferably 110-135° C., and the stretching ratio is 1.1-2, preferably 1.2-1.8.

[0038] In step S4, the temperature of the longitudinal heat setting equipment is 80-100°C, preferably 95°C, and the stretching ratio is 0.8-1, preferably 0.95. After longitudinal heat setting, it is preferred to also perform winding, slitting and aging treatment. The winding method is a low-tension, roller-approaching winding method, and there are no special restrictions. The rollers used for winding are all large-diameter carbon fiber rollers. The diameter of the winding roller is 500-1000mm, preferably 600-800mm. Low-tension relaxation winding is used during the winding process to help reduce the poor surface appearance of the diaphragm due to stress.

[0039] Stretching orients the molecular chains, improving the mechanical strength of the product and adding new properties. After simultaneous longitudinal and transverse stretching, the separator is oriented in both directions, allowing the pore-forming agent to be more evenly distributed throughout the separator, resulting in uniform micropores and improved mechanical properties in all directions. Example

[0040] 1.1 Polyethylene with a number average molecular weight of 1.5 million g / mol was fed into a pre-mixing tank at a rate of 50 kg / h, white oil was heated to 95°C and mixed with IRGANOX 1010, and the pre-mixing tank was fed with 200 kg / h for heating and stirring with polyethylene. The mass ratio of polyethylene to white oil was 1:4, and the mass of IRGANOX 1010 was 0.6% of the mass of white oil. The mixture was mixed and stirred for 2 hours, and the mixture was injected into a twin-screw compounding extruder through a stirring feeding tank. The temperature inside the twin-screw compounding extruder was 230°C, and the extrusion temperature was 205°C. The extruded material was passed through a 12°C water bath and a 15°C cooling roller and the gap between the extrusion roller to be extruded into a thick sheet with a thickness of 2.4 mm.

[0041] 1.2 The thick sheet obtained in 1.1 is sequentially subjected to longitudinal stretching and biaxial simultaneous stretching. The longitudinal stretching temperature is 105°C and the stretching ratio is 1.3. The biaxial simultaneous stretching temperature is 126°C and the longitudinal stretching ratio is 7.5. The transverse stretching ratio is 15. After extraction and drying with dichloromethane, a semi-finished diaphragm is obtained.

[0042] 1.3 The semi-finished diaphragm obtained in 1.2 was subjected to a second transverse stretching at a temperature of 133° C. and a stretching ratio of 1.5 to obtain a middle diaphragm.

[0043] 1.4 The intermediate separator obtained in 1.3 is subjected to longitudinal heat setting treatment at a temperature of 95°C and a stretching ratio of 0.95 to obtain a lithium battery separator. The separator is then rolled and slit.

[0044] The battery separator obtained in 1.4 was tested for air permeability using an air permeability tester to obtain its air permeability. The results are shown in Table 1.

[0045] The tensile strength of the battery separator obtained in 1.4 was tested using a tensile tester to obtain its tensile strength in the transverse direction (TD) and longitudinal direction (MD). The results are shown in Table 1.

[0046] The battery separator obtained in 1.4 was tested for puncture strength and elongation using an electronic tensile testing machine to obtain its puncture strength and transverse (TD) and longitudinal (MD) elongation. The results are shown in Table 1.

[0047] Using the formula: 1-base film surface density (g / m 2 ) / base film thickness (um) / theoretical density (g / cm 3 The theoretical density is the actual density of PE particles, which is 0.95g / cm 3 .

[0048] The porosity of the battery separator obtained in 1.4 was tested, and the results are shown in Table 1.

[0049] The battery separator obtained in 1.4 was tested using a battery separator thermal shrinkage tester to obtain its thermal shrinkage. The results are shown in Table 1. Example

[0050] 2.1 Polyethylene with a number average molecular weight of 1.5 million g / mol was fed into a pre-mixing tank at a rate of 50 kg / h, white oil was heated to 95°C and mixed with IRGANOX 1010, and the pre-mixing tank was fed with polyethylene at a rate of 200 kg / h for heating and stirring. The mass ratio of polyethylene to white oil was 1:4, and the mass of IRGANOX 1010 was 0.6% of the mass of white oil. The mixture was mixed and stirred for 2 hours, and the mixture was injected into a twin-screw compounding extruder through a stirring feeding tank. The temperature inside the twin-screw compounding extruder was 230°C, and the extrusion temperature was 205°C. The extruded material was passed through a 12°C water bath and a 15°C cooling roller and the gap between the extrusion roller to be extruded into a thick sheet with a thickness of 2.3 mm.

[0051] 2.2 The thick sheet obtained in 2.1 is sequentially subjected to longitudinal stretching and biaxial simultaneous stretching. The longitudinal stretching temperature is 105°C and the stretching ratio is 1.5. The biaxial simultaneous stretching temperature is 126°C and the longitudinal stretching ratio is 7.5. The transverse stretching ratio is 12. After extraction and drying with dichloromethane, a semi-finished diaphragm is obtained.

[0052] 2.3 The semi-finished diaphragm obtained in 2.2 was subjected to a second transverse stretching at a temperature of 133° C. and a stretching ratio of 1.5 to obtain a middle diaphragm.

[0053] 2.4 The middle separator obtained in 2.3 is subjected to longitudinal heat setting treatment at a temperature of 95°C and a stretching ratio of 0.95 to obtain a lithium battery separator. The separator is then rolled and slit.

[0054] The battery separator obtained in 2.4 was tested for air permeability using an air permeability tester to obtain its air permeability. The results are shown in Table 1.

[0055] The tensile strength of the battery separator obtained in 2.4 was tested using a tensile tester to obtain its tensile strength in the transverse direction (TD) and longitudinal direction (MD). The results are shown in Table 1.

[0056] The battery separator obtained in 2.4 was tested for puncture strength and elongation using an electronic tensile testing machine to obtain its puncture strength and transverse (TD) and longitudinal (MD) elongation. The results are shown in Table 1.

[0057] Using the formula: 1-base film surface density (g / m 2 ) / base film thickness (um) / theoretical density (g / cm 3 The theoretical density is the actual density of PE particles, which is 0.95g / cm 3 .

[0058] The porosity of the battery separator obtained in 2.4 was tested, and the results are shown in Table 1.

[0059] The battery separator obtained in 2.4 was tested using a battery separator thermal shrinkage tester to obtain its thermal shrinkage. The results are shown in Table 1. Example

[0060] 1.1 Polyethylene with a number average molecular weight of 1.5 million g / mol was fed into a pre-mixing tank at a rate of 50 kg / h, white oil was heated to 95°C and mixed with IRGANOX 1010, and the pre-mixing tank was fed with 200 kg / h for heating and stirring with polyethylene. The mass ratio of polyethylene to white oil was 1:4, and the mass of IRGANOX 1010 was 0.6% of the mass of white oil. The mixture was mixed and stirred for 2 hours, and the mixture was injected into a twin-screw compounding extruder through a stirring feeding tank. The temperature inside the twin-screw compounding extruder was 230°C, and the extrusion temperature was 205°C. The extruded material was passed through a 12°C water bath and a 15°C cooling roller and the gap between the extrusion roller to be extruded into a thick sheet with a thickness of 2.4 mm.

[0061] 1.2 The thick sheet obtained in 1.1 is sequentially subjected to longitudinal stretching and biaxial simultaneous stretching. The longitudinal stretching temperature is 105°C and the stretching ratio is 1.5. The biaxial simultaneous stretching temperature is 126°C and the longitudinal stretching ratio is 7.5. The transverse stretching ratio is 12. After extraction and drying with dichloromethane, a semi-finished diaphragm is obtained.

[0062] 1.3 The semi-finished diaphragm obtained in 1.2 was subjected to a second transverse stretching at a temperature of 133° C. and a stretching ratio of 1.45 to obtain a middle diaphragm.

[0063] 1.4 The intermediate separator obtained in 1.3 is subjected to longitudinal heat setting treatment at a temperature of 95°C and a stretching ratio of 0.95 to obtain a lithium battery separator. The separator is then rolled and slit.

[0064] The battery separator obtained in 1.4 was tested for air permeability using an air permeability tester to obtain its air permeability. The results are shown in Table 1.

[0065] The tensile strength of the battery separator obtained in 1.4 was tested using a tensile tester to obtain its tensile strength in the transverse direction (TD) and longitudinal direction (MD). The results are shown in Table 1.

[0066] The battery separator obtained in 1.4 was tested for puncture strength and elongation using an electronic tensile testing machine to obtain its puncture strength and transverse (TD) and longitudinal (MD) elongation. The results are shown in Table 1.

[0067] Using the formula: 1-base film surface density (g / m 2 ) / base film thickness (μm) / theoretical density (g / cm 3 The theoretical density is the actual density of PE particles, which is 0.95g / cm 3 .

[0068] The porosity of the battery separator obtained in 1.4 was tested, and the results are shown in Table 1.

[0069] The battery separator obtained in 1.4 was tested using a battery separator thermal shrinkage tester to obtain its thermal shrinkage. The results are shown in Table 1.

[0070] Table 1

[0071] The results show that, while maintaining the same basic formula, the three examples only varied the sheet thickness or the draw ratio. The draw ratio showed a greater impact, increasing it to improve the puncture strength of the battery separator. Other physical properties, which were correlated with thickness, showed little change. All three examples meet the typical power battery separator requirement of a puncture strength of ≥400 gf and are also expected to meet the expected growth in battery industry demand.

[0072] In summary, the present invention provides a method for preparing a high-strength, ultra-thin battery separator. The prepared battery separator has high strength, ultra-thin thickness, and uniform thickness and micropore size distribution.

[0073] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a high-strength, ultra-thin battery separator, characterized in that: The steps include: S1: A mixture of polyolefin resin, pore-forming agent and additives in proportion is weighed and placed in a storage tank, and the storage tank containing the pore-forming agent is heated. The materials are then added to a stirring tank for heating and pre-mixing. The mixed materials are then sent to a feeding tank, and then sent to a twin-screw compounding extruder through the feeding tank for extrusion. The extruded materials are accumulated between a cooling roller with a water bath and an adjustable roller, and then extruded into a thick sheet through the gap between the cooling roller and the adjustable roller; S2: longitudinally stretching and biaxially stretching the obtained thick sheet in sequence, extracting and drying to obtain a semi-finished separator; S3: performing a second transverse stretching on the obtained semi-finished diaphragm to obtain a middle diaphragm; S4: performing longitudinal heat setting treatment on the obtained middle separator to obtain a lithium battery separator.

2. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: In step S1, the mass ratio of the polyolefin resin to the pore-forming agent is 1:(3-5), the mass of the additive is 0.3%-2.5% of the mass of the pore-forming agent, the heating temperature of the storage tank where the pore-forming agent is located is 95°C, the feeding flow rate of the polyolefin resin is 50 kg / h, the feeding flow rate of the mixture of the pore-forming agent and the additive is 200 kg / h, the stirring time of the stirring tank is 2 hours, the internal temperature of the twin-screw blending extruder is 200-260°C, the extrusion temperature is 200-260°C, the temperature of the water bath is 9-26°C, the temperature of the cooling roller is 7-30°C, and the thickness of the slab is 1-4 mm.

3. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: The polyolefin resin is an ultra-high molecular weight polyolefin resin of 1.5-2.3 million or a high-density polyolefin resin.

4. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: The pore-forming agent is phthalate, phosphate or white oil.

5. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: The additive is one or more of an antioxidant, an antistatic agent or an inorganic filler.

6. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: In step S2, the temperature for longitudinal stretching is 90-130° C., the stretching ratio is 1.2-5.0, the temperature for bidirectional synchronous stretching is 100-130° C., the longitudinal stretching ratio is 6.5-7.5, the transverse stretching ratio is 10-16, and dichloromethane is used for extraction.

7. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: In step S3, the temperature of the second transverse stretching is 100-135° C., and the stretching ratio is 1.1-2.

8. The method for preparing a high-strength, ultra-thin battery separator according to claim 1, characterized in that: In step S4, the temperature of the longitudinal heat setting equipment is 80-100° C., and the stretching ratio is 0.8-1.

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